When telecom operators sell digital subscriber line (DSL) services for data transmission over a telecommunication line (or just “line” or “loop”) to a customer, it is often a problem that telecommunication line properties that affect DSL services are not sufficiently well known. Because of that, it may not be possible to accurately predict how much line capacity, such as a data transfer rate given in Mbits/second, the line can support and hence which capacity that can be sold to the customer.
An operator end of the line includes a Central Office (CO) at which a DSL Access Multiplexer (DSLAM) is typically located while a customer end of the line includes a customer premises (CP) at which a Customer Premises Equipment (CPE) is typically located. Other examples where a DSLAM can be located includes locations beyond the CO, such as an equipment cabinet or a space in a basement of an office building. The DSL service is set up between the CO and the CP where a downstream transmission refers to CO to CP (or DSLAM to CPE) transmission while upstream transmission refers to CP to CO (or CPE to DSLAM) transmission. The DSL Access Multiplexer in the CO is used for data transmission but can also be used for determining line properties such as attenuation which in combination with knowledge about transmitter power spectral density and noise power spectral density can be used for estimating line capacity.
When determining downstream line capacity accurately in a DSL system, the so called far-end noise must be known, i.e. in this case the noise that would be experienced by a CPE connected to the line. In noise-limited systems, far-end noise can be approximated by knowing the receiver noise (noise at the CPE) or similar prior knowledge. For example, a value of −140 dBm/Hz is often used when assuming far-end noise for a DSL CPE. However, in reality many lines for DSL communication are limited by crosstalk (interference) from adjacent lines.
The crosstalk typically comprises Far End Crosstalk (FEXT) and Near End Crosstalk (NEXT), where FEXT is interference between two adjacent lines, as measured at the end of the line furthest from the transmitter, and NEXT is interference between adjacent lines, as measured at the end of the line nearest to the transmitter. Since a transmitter or transceiver is arranged in each of the DSLAM and CPE, a respective NEXT and FEXT can be determined for each of the DSLAM and the CPE. Since most DSL technologies separate frequency bands for upstream and downstream communication, NEXT is usually not a problem while FEXT is a major cause of line capacity limitation.
Accordingly, determining the FEXT is important when assessing the line capacity of a DSL line, and various techniques for determining FEXT exist, such as i) double-ended line test methods that measure noise at both sides, i.e. both upstream FEXT at the DSLAM and downstream FEXT at the CPE, ii) estimation methods for determining downstream FEXT from DSLAM NEXT, iii) methods for interpolation/extrapolation of measured FEXT couplings to other frequencies and to the opposite transmission direction using reciprocity, iv) methods using extrapolation of measured FEXT level to higher frequencies in the same transmission direction as for the FEXT measurements, v) methods using an average upstream noise level as an estimate of the far-end noise level for all downstream frequencies, and vi) methods employing a 1% worst-case FEXT models, without any input regarding the current noise situation.
More specific examples of techniques relating to determining parameters affecting line capacity can be found in patent documents WO2008/030145, WO2005/114861, US20090092036, US20050057880A1 and WO2008/008015.
The techniques for determining FEXT described above are generally capable of estimating FEXT-values. However, due to various deficiencies the estimated FEXT-values tend to suffer from an inability to accurately reflect a true FEXT-value that may indicate e.g. the true far end noise and/or true line capacity, in particular when no equipment is allocated at the customer premises.